The present application relates to nuclear magnetic resonance (NMR) logging of subterranean formations.
Well logging is a common practice in the oil and gas industry to evaluate underground formations for the presence and producibility of subterranean formations. Among the most important parameters determined in the process are the depth and thickness of formation layers containing hydrocarbon, the formation porosity (i.e., the relative amount of void space in the formation), the hydrocarbon saturation (i.e., the relative percentage of hydrocarbons versus water in the pore space), and the permeability of the formation (i.e., the ability of the oil, gas, or water to flow out of the formation, into the well and eventually to the surface for recovery).
Presently, NMR logging is considered to be one of the most effective techniques for determining these geologic parameters. NMR technology has many advantages over other logging techniques (such as gamma ray logging, sonic logging, electric logging, and others), one of the most significant being the independence of NMR measurements from formation lithology. In particular, NMR data relates in a simple manner to formation pore sizes. This relationship facilitates detection of formation fluids (e.g., gas, oil, and water) independent of the matrix mineralogy. To this end, in addition to estimation of formation porosity, hydrocarbon saturation, and permeability, NMR logging enables computation of clay-bound water, capillary-bound water, and free fluid volumes, which aid in comprehensively evaluating the subterranean formation.
NMR logging used for evaluating subterranean formations typically includes a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, which is a sequence of radio-frequency (RF) pulses producing NMR spin echoes that decay with time. The decay time is used to calculate the NMR relaxation data (e.g., the spin-spin relaxation time (T2)). The CPMG pulse sequence comprises one excitation RF pulse and a plurality of refocusing RF pulses. The refocusing RF pulses in the plurality of refocusing pulses of the CPMG pulse sequence are spaced by a time interval (TE). Nuclear spin echoes produced by the refocusing pulses are spaced by the same time interval TE, which is referred to as “echo spacing.” Therefore, it is advantageous to reduce the TE to a minimum possible value in order to produce larger number of echoes per unit time and achieve higher signal to noise ratio (SNR) per unit time. One of the limiting factors in achieving shorter TE is the refocusing pulse width.
Most NMR logging use a gradient static magnetic field and employ multi-frequency measurements, which is referred to herein as gradient multi-frequency NMR. Gradient multi-frequency NMR techniques use CPMG pulse sequences at different frequencies to investigate different volumes of the subterranean formation at various radial distances from the wellbore, which in effect are concentric cylinders radiating from the wellbore. As used herein, the term “sensitive volume” refers to the volume of the formation investigated by NMR. The thickness of the sensitive volumes is determined by the bandwidth of the refocusing RF pulse.
Because gradient multi-frequency NMR techniques use a different excitation frequency to investigate each sensitive volume, multiple sensitive volumes can be interrogated while waiting for the nuclear magnetization in the first sensitive volume to recover its equilibrium state. As a result, the total signal acquired per unit time for the gradient multi-frequency NMR techniques may be increased by a factor of 5-10 over single-frequency techniques where each sensitive volume must be interrogated separately and sequentially. Additionally, the gradient multi-frequency NMR techniques may advantageously identify the formation fluid type and provide a profile of saturation in a single pass.
The multiple frequencies, typically 5-10, used in a gradient multi-frequency NMR technique to interrogate different sensitive volumes need to fit in an operating frequency range. This operating frequency range and, more specifically, the upper and lower operating frequencies define the volume of the formation that could potentially be interrogated. Maximum depth of investigation, for a given magnet configuration, determines the operating frequency range. Individual operating frequencies determine the distance of the corresponding sensitive volumes from the tool sensor. The farther the sensitive volume from the tool the less the signal induced in the NMR antenna.
One of the challenges of fitting the multiple frequency bands in the operating frequency range is the interference between adjacent frequency bands due to out-of-band parts of the RF pulse spectrum. The out-of-band parts interfere with adjacent frequency bands by distorting equilibrium state of nuclear magnetization in the neighborhood of the excitation band. To decrease the interference, the sensitive volumes interrogated are spaced apart, typically, by 3 times the sensitive volume thickness. Accordingly, a significant portion of the formation volume around the wellbore is not investigated and analyzed.
One strategy for addressing interference and increasing the volume interrogated is to change the shape of the RF pulses, which essentially suppresses the out-of-band parts of the RF pulse spectrum. In some instances, the shape of the RF pulses is changed from simple rectangular pulses to complex non-rectangular pulses (e.g., Hann pulses). However, using pulse-shaping techniques only increases the volume of the formation interrogated to about 30-50% of the volume that could be interrogated as defined by the operating frequency range.